Testing a container for leaks can be performed by evacuating the container being tested and spraying a test gas over it. Alternatively, if the container is small, it is also known to fill it with the test gas and place it in a test tank which is then evacuated. In either case, if the gas penetrates through the wall of the container, the container is leaky. A test gas detector connected to the vacuum line for evacuating the container or test tank thus makes it possible to determine whether such leakage is occurring. The test gas detector is often a mass spectrometer which is set for the test gas, preferably helium, and therefore for mass 4. Mass spectrometers, however, can be operated only at a pressure less than 1.times.10.sup.-3 mbar and, preferably of 1.times.10.sup.-4 mbar or less, while the pressure at which the leakage test can be performed is substantially greater. It is therefore known to pass test gas countercurrently through the high vacuum pump that evacuates the mass spectrometer (see, for example, Briggs U.S. Pat. No. 3,690,151, substantially corresponding to German No. 1,937,271).
In order to achieve high-capacity suction to the container being tested so as to reduce the pressure-adjustment time without reducing sensitivity, it is desirable to perform leakage tests at pressures which require the use of a high-vacuum pump also for the evacuation of the container being tested or the test tank, as the case may be. It is known from Becker U.S. Pat. No. 3,520,176 corresponding to German No. 1,648,648 and Schrader U.S. Pat. No. 3,616,680 corresponding to German No. 2,049,117 to use a turbomolecular pump having two, tandem pumping stages, one serving for the evacuation of the mass spectrometer and the other for the evacuation of the sample or the test vessel.
The container or test tank and the mass spectrometer, however, were both connected to the turbomolecular between the two tandem pump stages, while the forepump was connected to both outer ends of the two pump stages via a conduit system situated outside of the pump casing. This produced relatively great difficulties in separating the container or test tank connection from the mass spectrometer connection for the different pressures desired. In the Becker patent, a separate pump stage is required, with several rows of turbine buckets which are intended to form a gas-tight labyrinth packing. In the Schrader U.S. Pat. No. 3,616,680, the separation is said to be achieved by a "virtually gas-impermeable partition wall". The production of such a partition wall is technically complicated, because it is in the high vacuum and has to be sealed against a central shaft. Furthermore, in the previously known embodiments it is disadvantageous that the test gas that is to be registered has to travel a relatively long path, namely from the container or test tank connection through the stage serving for the evacuation of the test tank, through the forepump conduit system situated outside of the pump casing--partially in a counterflow--and finally in a counterflow through the pump stage serving for the evacuation of the mass spectrometer, all of which impairs the sensitivity of the leakage test.